>
> In general, Geordie (the traditional dialect spoken around the Tyne
> River in England) is considered to be the English dialect most difficult
> for North Americans.

To that I would add Glaswegian. When watching the
Scots-produced mystery shows that show up on PBS in the United
States on occasion, my wife and I often turn to each other
in bafflement and say, "Subtitles, please."

> In countries where English is widely spoken as a second language (former
> parts of the British Empire), the varieties are often very different.
> Indians have trouble with Kenyan English and vice versa, IIRC.

And in response to Elliotte Harold's comment, when encountering
spoken versions of English, one's task of understanding is often
made easier by the fact that an interlocutor will generally try to
move their pronunciation and usage towards what they (and you)
perceive as more standard, specifically to assist in the task
of communication. And varieties heard in media also tend to be
more comprehensible regional norms, precisely because they are
aiming at a wide audience.